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Advanced React: Optimization

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The React Profiler

The React profiler browser extension allows developers to record a session while using a React app in development mode. Once recorded, the profiler displays a flame graph, alongside other information on how React rendered each component.

The React profiler flame graph

A flame graph in the React profiler displays all components that were rendered during a session. Components that took longer to render are displayed in yellow, while ones that took less time to render are displayed in blue.

The React profiler displays a flame graph where the Profile component is displayed as a yellow block.


Memoization prevents rerunning expensive operations by returning a cached result when given the same inputs as a past operation.

Memoizing values

React provides the useMemo() hook to memoize values. It takes two arguments, a function that returns a value, and a list of dependencies. Given the same list of dependencies, useMemo() will return a memoized value.

Higher order components

A higher order component is a component that takes in a component, adds functionality, then returns a new component with extra functionality.

Memoizing React components

React.memo() memoizes a React component. It takes a React component as an argument, then when given the same props, it will prevent React from re-rendering the provided component even when its parent re-renders.

import React from 'react';
const MemoizedComponent = React.memo(() => {
return <div />

Memoizing functions

React provides the useCallback() hook to memoize functions. It takes two arguments, a function, and a list of dependencies. Given the same list of dependencies, useCallback() will return a memoized function.

import { useCallback } from 'react';
// ...
const memoizedFunction = useCallback((number) => formatNumber(number), []);

Code splitting

Code splitting is the act of separating code from the main JavaScript bundle in a React app to allow the main bundle to be smaller which usually reduces download, parsing, and execution time.


When a library or a component is split out from the main JavaScript bundle.js file, it is called a chunk.

Code splitting a module

The import() function imports a module while instructing the bundler to create a separate chunk for the imported module. import() takes one argument, which is the path of a module. import() returns a JavaScript Promise.

const largeModule = import('./largeModule.js');

Code splitting a component

The React.lazy() function imports a React component while instructing the bundler to create a separate chunk for the imported component. React.lazy() takes one argument, a function that returns an import(), which takes a path to a React component as its argument.

const LargeComponent = React.lazy(await () => import('./LargeComponent'));


The <Suspense> React component wraps components imported with React.lazy(). <Suspense> instructs React to load the rest of the React app and to display a loading state while the React component is downloaded and rendered. <Suspense> takes one prop named fallback, which is a React component shown while its lazily loaded children are unavailable.

import { Suspense }, React from 'react';
const LargeComponent = React.lazy(await () => import('./LargeComponent'));
const Page = () => {
return {
<Suspense fallback={<p>Loading...</p>}>
<LargeComponent />